Spacecraft are subjected to a wide range of thermal environments during service. For example, one side of the spacecraft may face in a direction away from the sun, while another side faces toward the sun. Thermal control is desirable because heat is radiated into space, which cools the spacecraft, but the spacecraft can simultaneously be heated intensively in direct sunlight.
Active and passive temperature control techniques are used to maintain the interior temperature of the spacecraft, which contains persons or sensitive instruments, within acceptable operating limits. Active temperature control may involve machinery or electrical devices, such as electrical heaters, electrical coolers, and heat pipes. In contrast, passive temperature controls are techniques that do not involve machinery or electrical devices, but may include thermal coatings or structural designs.
Specifically, one known approach to passive temperature control includes use of surface coatings, typically termed “paints”, on the external surface of the spacecraft. A white paint, for example, has a low solar absorptance, while a black paint has a high solar absorptance. Selective application of such paints to various elements of the spacecraft exterior greatly aids in controlling its temperature.
In addition to passive temperature control, it is desirable for paint applied to the surface of spacecraft to dissipate electrostatic charges (i.e., provide electrostatic dissipation (ESD)) that may develop along the external surface of the spacecraft. The electrostatic charges may accumulate and cause arcing and possible damage to, or interference with, sensitive electronic equipment on or in the spacecraft. In order to dissipate electrostatic charge, the paint must have at least some electrical conductivity. Specifically, it is desirable that coatings capable of electrostatic dissipation (ESD) have a surface resistivity of less than about 109 ohms per square.
In addition to thermal control and ESD, paint for use on spacecraft and spacecraft components should exhibit additional characteristics for spacecraft applications. For example, the paint should be stable during long-term service in a space environment. The paint is desirably moderately tough and flexible so that it does not crack and flake away as it is flexed due to mechanical or thermal strains.
A number of white, electrostatic-dissipative paints are known for spacecraft use. One of the known paints includes an inorganic potassium silicate binder. The paint having the potassium silicate binder typically has a solar absorptance of from about 0.13 to about 0.15, see U.S. Pat. No. 5,094,693, whose disclosure is incorporated by reference in its entirety. However, it is desirable to have paints with lower solar absorptance. The lower the value of the solar absorptance, the lower the heating of the paint and thence the underlying substrate, in the intense heating of direct sunlight.
Known white thermal paints have high production costs. For example, doping of materials involves labor-intensive and expensive processes and expensive materials.
What is needed is an improved white thermal-control paint that is operable and stable in a space environment, which has a lower solar absorptance than available in existing paints, which has a lower operating temperature-limit than existing paints, which can manage electrostatic discharge (ESD) and can be manufactured inexpensively. In addition, it is desired to have an inorganic white thermal control coating which; a) minimizes space radiation degradation over time, b) minimizes the beginning of life (BOL) and the end of life (EOL) solar absorptance (α), c) maximizes the infrared emissivity (e), while at the same time d) maximizing the electrostatic dissipative (ESD) properties at low temperatures (below −65° C.). The present disclosure fulfills this need, and further provides related advantages.